Contract

saints1996

Hello,
Three years ago my sister in law bought a static caravan and left it on a site year round. It was agreed that a site fee of £4000 most be paid every march in order for the caravan to remain on the site and for the last three years they have paid this fee without question.
However in December 2012 they decided they wanted to sell the caravan and the company who owns the site agreed to find a buyer for them for a fee of course. Therefore they informed the company that come March if the caravan wasn't sold they would just pay the fees on a month by month basis due to the fact they wouldn't be using the caravan and intended to sell ASAP. This was all OK with the land owner and both parties have been awaiting a buyer.
However when my sister in law rang up at the end of February to check if there had been any interest with the caravan she was informed the company had been sold to new owners and she had to pay £4000 by the 1st March 2013. Therefore she went on to explain what had been agreed and was told tough pay the £4000 or else we will get the bailiffs involved.
She's informed the manager that she hasn't got the money due to intending on paying month by month. Also when asked if they did pay the £4000 but if the caravan sold the month after would they get 11 months money back she was told no.

Therefore i was hoping someone could give me some advice in how to go the best way about this? They are willing to work with the new owners but were never informed there would be new owners and these people seem to expect the money or else it seems. Does my sister in law have any rights.

Thanks